1. Field of the Invention
The present invention relates to a flat cable for connecting a plurality of circuits or devices such as computers and communication devices and, particularly, to a shielded flat cable having a shielding layer which is made conductive with an earth cable.
2. Description of the Prior Art
In recent years, a flat cable 50 shown in FIGS. 3(a) and 3(b) has been frequently used as a connection cable for connecting devices such as computers and communication devices in place of a coaxial cable. This flat cable 50 is produced by sheathing a plurality of conductors 22 disposed in parallel to one another with an insulating member 21A except both end portions thereof, and the conductors 22 consist of a large number of signal lines 22S and at least one ground line 22G.
As is already known, a coaxial cable has excellent electromagnetic wave shielding properties as a ground line surrounds signal lines. However, in the case of the above flat cable 50, as the signal lines 22S and the ground line 22G are arranged in parallel to one another, noise is readily superimposed on the above signal lines 22S.
As a flat cable used for connecting computers and communication devices which require resistance to noise, a shielded flat cable 10A comprising an electromagnetic wave shielding layer 13 for preventing malfunctioning caused by the above noise as shown in FIGS. 4(a) to 4(c) is widely used. This shielded flat cable 10A is produced by sheathing the same cable body 11 as that of the above flat cable 50 with a film-like shielding member 14 having a shielding layer 13 made of a metal film formed on one side of an insulating substrate 12 in order to protect the above plurality of conductors 22 from an electromagnetic wave. The above cable body 11 and the shielding member 14 are bonded together by a conductive adhesive. A non-insulating portion 16 devoid of the above insulating member 21A is formed on part of the above ground line 22G, and the above shielding layer 13 and the above ground line 22G are made conductive with each other by the above conductive adhesive constituting an adhesive layer 15 in the above non-insulating portion 16. Therefore, external noise is caused to flow from the above shielding layer 13 to the ground (earth) of a device body through the above ground line 22G, thereby making it possible to ensure the electromagnetic wave shielding properties of the flat cable 10A.
As the above conductive adhesive, there is disclosed a conductive adhesive prepared by dispersing silver particles or copper particles as a conductive material in an adhesive resin essentially composed of a thermoplastic resin having heat-sealing properties, such as polyethylene, polyester or polyamide (Japanese Laid-open Patent Application No. 7-94036). However, when silver particles are used, the cost is boosted though excellent conductivity is obtained. When copper particles are used, electromagnetic wave shielding properties deteriorate along the passage of time.
As means of solving the above problems, there is proposed a method of obtaining stable electromagnetic wave shielding properties at a low cost by using an adhesive comprising a nickel filler in place of silver particles or copper particles (Japanese Laid-open Patent Application No. 11-120831).
The conductor pattern of the cable body 11 is becoming finer and finer along with the downsizing of a device, thereby increasing the resistance values of the above conductors 22 and generating heat easily with the result that the heat resistance requirement for the shielded flat cable 10A is becoming higher and higher. Further, the shielded flat cable 10A is required to improve adhesion between the cable body 11 and the shielding member 14 in order to ensure connection reliability in a high-temperature and high-humidity condition in addition to the above heat resistance.
However, as the conductive adhesive used in the adhesive layer 15 of the above conventional shielded flat cable 10A is an adhesive resin essentially composed of a thermoplastic resin, it has problems with heat resistance and the reliability of connection strength in a high-temperature and high-humidity condition.
It is an object of the present invention which has been made in view of the above problems of the prior art to provide a shielded flat cable which has excellent heat resistance and can ensure connection reliability even in a high-temperature and high-humidity condition.
According to a first aspect of the present invention, there is provided a shielded flat cable comprising a cable body in which a plurality of conductors including at least one ground line is covered except at least part of the ground line with an insulating member, a shielding member having a shielding layer made of a conductive material formed on one side of an insulating substrate for sheathing the cable body, and an adhesive layer comprising an adhesive with conductive particles dispersed therein, part of the adhesive layer being in contact with a non-covered portion of the ground line for bonding the shielding member to the cable body, wherein the adhesive is a thermally or optically curable adhesive comprising a resin having heat resistance and flexibility after curing as a base resin.
According to a second aspect of the present invention, there is provided a shielded flat cable, wherein the resin is an ethylene-vinyl acetate copolymer or a copolymer of a monomer and at least one of ethylene and vinyl acetate.
According to a third aspect of the present invention, there is provided a shielded flat cable, wherein the resin is a polymer obtained by acetalizing a polyvinyl alcohol.
According to a fourth aspect of the present invention, there is provided a shielded flat cable, wherein the content of the acetal group in the polymer is 30 mol % or more.
According to a fifth aspect of the present invention, there is provided a shielded flat cable, wherein the resin is a polymer obtained by acetalizing a polyvinyl alcohol, or an acrylic resin obtained by polymerizing at least one of an acrylic monomer and a methacrylic monomer.
According to a sixth aspect of the present invention, there is provided a shielded flat cable, wherein the resin is a polyester unsaturated compound soluble in a solvent.
According to a seventh aspect of the present invention, there is provided a shielded flat cable, wherein the base resin is mixed with a phosphoric acid methacrylate and melamine-based resin.
According to an eighth aspect of the present invention, there is provided a shielded flat cable, wherein the phosphoric acid methacrylate is used in an amount of 0.1 to 60 parts by weight and the melamine-based resin is used in an amount of 0.1 to 200 parts by weight based on 100 parts by weight of the base resin.
According to a ninth aspect of the present invention, there is provided a shielded flat cable, wherein the phosphoric acid methacrylate is one or more of 2-methacryloyloxyethyl acid phosphate and diphenyl-2-methacryloyloxyethyl phosphate.
According to a tenth aspect of the present invention, there is provided a shielded flat cable, wherein the melamine-based resin is one or more of melamine resin, isobutylated melamine resin, butylated melamine resin and methylated melamine resin.
According to an eleventh aspect of the present invention, there is provided a shielded flat cable, wherein the adhesive comprises an organic peroxide or optical sensitizer in an amount of 0.1 to 10 parts by weight based on 100 parts by weight of the base resin.
According to a twelfth aspect of the present invention, there is provided a shielded flat cable, wherein the adhesive comprises at least one reactive compound selected from the group consisting of an acryloxy group-containing compound and methacryloxy group-containing compound in an amount of 0.5 to 80 parts by weight based on 100 parts by weight of the base resin.
According to a thirteenth aspect of the present invention, there is provided a shielded flat cable, wherein the adhesive comprises a silane coupling agent in an amount of 0.01 to 5 parts by weight based on 100 parts by weight of the base resin.
According to a fourteenth aspect of the present invention, there is provided a shielded flat cable, wherein the adhesive comprises a hydrocarbon resin in an amount of 1 to 200 parts by weight based on 100 parts by weight of the base resin.
According to a fifteenth aspect of the present invention, there is provided a shielded flat cable, wherein the amount of the conductive particles is 1 to 70 parts by weight based on 100 parts by weight of the base resin.
According to a sixteenth aspect of the present invention, there is provided a shielded flat cable, wherein the average particle diameter of the conductive particles is 0.1 to 100 xcexcm.
According to a seventeenth aspect of the present invention, there is provided a shielded flat cable, wherein a metal filler is used as the conductive particles.
According to an eighteenth aspect of the present invention, there is provided a shielded flat cable, wherein nickel powders are used as the metal filler.
According to a nineteenth aspect of the present invention, there is provided a shielded flat cable, wherein a flame retardant film is used as the substrate of the shielding member.
Other objects, advantages and features of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings.